Superconducting generators for large direct-drive wind turbines
This thesis further improves upon the original 10 MW design of the double claw pole generator, by systematically addressing its shortcomings. The original design is a fully iron cored machine with a stationary superconducting field winding. Its structure allows it to be highly modular, reliable and cost-effective. Its main disadvantages are, when compared to other superconducting generator designs, its weight and efficiency. The double claw pole generator is a large diameter iron-cored axial-flux machine, due to these features a very stiff mechanical structure is required to main the air gap clearances, which leads to a very heavy structural mass. A novel stator design is introduced, which partially deviates the air gap closing forces into the radial direction, reducing the axial component of forces. This enabled the structural mass to be reduced from 126 tonnes to 115 tonnes. Secondly, the field core of the double claw pole machine was replaced by an inner stator. The additional stator increases the electric loading of the machine while also further increasing its modularity and improving the generator efficiency. With a target efficiency of 95 %, the power output of the generator was increased from 10 MW to 11.5 MW, while maintaining the same machine diameter and axial length. To further increase the power density and modularity, the possibility of stacking machine modules was explored. Stacking two standardised modules concentrically was found to result in a smaller and lighter machine than the original design. Additionally, the standardised modules, due to their smaller size, greatly simplify the transportation of the generator. The addition of the inner stator was found to be a very promising design. It improves the original concept of the machine in terms of power density, efficiency and modularity. It is believed that this makes the design even more competitive in the high-temperature superconducting generator market. Finally, detailed electromagnetic modelling of superconductors was performed in the electromagnetic environment relevant to electrical machines. Particular focus was put on the dynamic loss mechanisms in superconducting field windings. Through the new knowledge gained on the loss characteristics, the cooling requirements can be better understood, potentially increasing the reliability of superconducting windings and their associated cooling systems.